Thomas Young, in the early 1800s, showed that a broad range of colors can be generated by mixing three beams of light, provided their frequencies were widely separately.
When three such beams combine to produce white light, there are called primary colors.
There is no single unique set of these primaries, nor do they have to be monochromatic.
The three components (emitted by three phoshors) that generate the whole gamut of hues seen on a color TV set are Red, Green, Blue.

Looking through a colored window or cloth is another story.
Yellow cloth, paper, dye, paint, and ink all selectively absorb blue and reflect what remains - yellow - and that is why they appear yellow.
This java applet let you play with mixing light beams and paint pigments.

Click the right mouse button twice quickly to switch between two different modes:

Mixing light beamsblack
Mixing paint pigmentswhite

Click left mouse button and drag one of the colored oval to move it around.

Enter RGB values into the text field to change the color of the selected oval.
The color code (RGB value) at the mouse tip are shown in colored background.